LONDON — Police thwarted a devastating terrorist plot yesterday, discovering two Mercedes-Benzes loaded with nails packed around canisters of propane and gasoline set to detonate and kill possibly hundreds in London’s crowded theater and nightclub district.
The plot, coming only two days after Gordon Brown took over as prime minister, recalled the attacks on July 7, 2005, when the London Underground and an iconic double-decker bus were targeted by a group of homegrown suicide bombers who killed 52 persons.
As police searched for car bombs and terrorists in the city of 7.5 million, roads were closed and police sirens wailed. Authorities stepped up security across Britain, from Central London streets to the Wimbledon tennis tournament.
The first car bomb, found near Piccadilly Circus, was powerful enough to have caused “significant injury or loss of life” at a time when hundreds were in the area, British anti-terror police chief Peter Clarke said.
Mr. Clarke said yesterday evening that the second car — another Mercedes — was originally parked illegally on nearby Cockspur Street, but had been towed from the West End to an impound lot near Hyde Park.
“The vehicle was found to contain very similar materials to those that had been found in the first car,” he said. “There was a considerable amount of fuel and gas canisters. As in the first vehicle, there was also a quantity of nails. This, like the first device, was potentially viable.”
There was no immediate claim of responsibility for the bombs.
“We are currently facing the most serious and sustained threat to our security from international terrorism,” Britain’s new home secretary, Jacqui Smith, said after an emergency meeting of top officials.
Police were examining footage from closed-circuit TV cameras, Mr. Clarke said, hoping the surveillance network that covers much of Central London will help them track down the drivers of the cars.
Rep. Peter T. King, New York Republican and the ranking member of the House Homeland Security Committee who was briefed on the investigation, said British authorities had recovered a cell phone that they thought was to be the trigger for the explosion.
“They found a cell phone and it was going to be used to detonate the bomb,” he said.
The U.S. government urged Americans abroad to be vigilant but officials said they saw no potential terrorist threat in the United States ahead of the July Fourth holiday. Department of Homeland Security Secretary Michael Chertoff said there were no immediate plans to raise the U.S. national threat level, currently at yellow, meaning elevated.
New York strengthened its already tight security as a precaution, putting additional police in Times Square and the mass transit system.
The events unfolded when an ambulance crew — responding to a call just before 1:30 a.m. about a person who had fallen at a Haymarket nightclub — noticed smoke coming from a car parked in front of the building, he said.
The crew alerted police, and a bomb squad manually disabled the device, Mr. Clarke said.